1. Field of the Invention
The present invention relates to a developing device for use in electrophotography. In addition, the present invention also relates to an image forming method and a process cartridge using the developing device.
2. Discussion of the Background
In electrophotography, an image is typically formed as follows:
(1) an image bearing member (hereinafter a photoconductor, a photoreceptor, a latent image bearing member, etc.) is charged and irradiated to form an electrostatic latent image thereon;
(2) the electrostatic latent image is developed with a toner to form a toner image;
(3) the toner image is transferred onto a recording medium such as a transfer paper; and
(4) the toner image is fixed on the recording medium by a heat roller.
Developing methods for use in electrophotography are classified into two-component developing methods using a two-component developer including a toner and a carrier and one-component developing methods using a one-component developer including a toner and no carrier. The two-component developing method is capable of stably producing relatively high quality images. However, there are disadvantages that the carrier easily deteriorates and the mixing ratio of the carrier and the toner largely varies with time. As a result, high quality images cannot be stably produced for a long period of time. The two-component developing method has another disadvantages of having poor maintainability and being large in size. For the above reasons, the one-component developing method has received attention recently.
In the one-component developing method, a toner (i.e., a one-component developer) is transported by at least one toner transport member (hereinafter a developing roller, a toner bearing member, etc.), and develops an electrostatic latent image formed on an image bearing member to form a toner image. A toner layer thickness control member is provided facing the toner transport member. The toner is charged when passing through the toner transport member. It is proposed that the toner layer thickness control member, a blade or a roller, applying pressure to a toner transported onto the toner transport member by facing the toner transport member to control the thickness of the toner layer.
The amount of the toner included in the toner layer formed on the toner transport member, for example, a developing roller, largely varies before and after the toner layer passes through the toner layer thickness control member, depending on the shape and fluidity of the toner. As a result, the charge quantity of the toner also largely varies, and image noises such as fog and density unevenness tend to be caused in the resultant image. Therefore, the toner layer needs to include a constant amount of toner and have a constant charge quantity as much as possible.
For the above reasons, various attempts have been made to improve developing rollers and toners. For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2006-145956 discloses a developing roller having specific surface roughness properties such as mean spacing of profile irregularities (Sm) and ten-point mean roughness (Rz) and a specific hardness. The ten-point mean roughness (Rz) represents variation in surface roughness of a developing roller, because the ten-point mean roughness (Rz) is the sum of the average height among 5 tallest peaks and the average depth among 5 lowest valleys. However, to transport a toner through the use of concavities on the surface of a developing roller, the degree of surface roughness (i.e., concavity and convexity) of the developing roller may be determined according to the average diameter of the toner, otherwise the toner cannot be stably charged and transported.
JP-A 2003-15401 also discloses a developing roller having specific surface roughness properties such as ten-point mean roughness (Rz), maximum peak (Ry), arithmetical mean roughness (Ra), and mean spacing of profile irregularities (Sm). However, the developing roller has too large a mean spacing of profile irregularities (Sm) to effectively transport a toner by embedding the toner in the concavities on the surface of the developing roller.
JP-A 2002-304053 also discloses a developing roller having specific surface roughness properties such as mean spacing of profile irregularities (Sm) and ten-point mean roughness (Rz). However, as mentioned above, to transport a toner through the use of concavities on the surface of a developing roller, the degree of surface roughness (i.e., concavity and convexity) of the developing roller may be determined according to the average diameter of the toner, otherwise the toner cannot be stably charged and transported.
JP-A 2003-207967 also discloses a developing roller having specific surface roughness properties such as mean spacing of profile irregularities (Sm) and ten-point mean roughness (Rz). However, the developing roller has too large a mean spacing of profile irregularities (Sm) to effectively transport a toner by embedding the toner in the concavities on the surface of the developing roller.
However, the above techniques are not satisfactory to improve chargeability and transportability of the toner. Therefore, the occurrence of fog and density unevenness in the resultant image cannot be prevented.